Hydrocarbon pyrolysis



United States Patent 3,536,776 HYDROCARBON PYROLYSIS Mou Neng M. Lo,Philadelphia, Pa., assignor to Mobil Oil Corporation, a corporation ofNew York No Drawing. Filed Aug. 24, 1967, Ser. No. 662,896 Int. Cl. C07c3/00; C10b 1/00 US. Cl. 260-683 16 Claims ABSTRACT OF THE DISCLOSUREHigh temperature hydrocarbon conversion reactions displaying a strongtendency toward carbon formation, as exemplified by the thermal crackingof hydrocarbons, are carried out in a reaction chamber constructed of orlined with a metal-ceramic material containing particles of acatalytically inert, refractory solid material (e.g., aluminum oxide)dispersed in chromium to minimize the deposition of carbon on equipmentsurfaces and to substantially eliminate the carburization of suchsurfaces.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to the conversion of hydrocarbons at elevated temperatures andmore particularly to the vapor phase cracking of hydrocarbons intoproducts having fewer carbon atoms per molecule than the materialcharged and/or a higher content of unsaturated hydrocarbons than thecharge. In one specific embodiment, it is concerned with an improvedprocedure for the production of ethylene in high yields by thenoncatalytic cracking of hydrocarbons at relatively high temperaturesand extremely short reaction times.

Description of the prior art The thermal cracking of hydrocarbonsderived from petroleum sources has long been practiced for a variety ofpurposes including the conversion of heavy feedstocks into more volatileliquid hydrocarbons suitable for use as motor fuel components and morerecently for the production of olefins and aromatic hydrocarbons as rawmaterials for the chemical industry. In thermal cracking, coke formationhas always been a problem as carbon deposits inside the cracking tubesor coils reduce the heat transfer through the tube walls and alsorestrict the flow of the reaction mixture through the tubes with aconsequently increasing pressure drop until the equipment is plugged.There is usually a concomitant carburizing action or destructive attackon the internal surfaces of the cracking tubes which weakens the metal.In addition, the conversion of part of the charge stock into carbonreduces the yield of the desired products.

The various expedients adopted for handling this coking problem haveusually involved compromises which include some undesirable features.Frequent decoking of the equipment reduces the amount of productive oronstream time; and frequent shutdowns for the replacement of badlycarburized tubes have the same effect. Lowering the reaction severity,e.g., by lowering the reaction temperature, results in lower productyields along with lower coke formation. Increasing the diameter of thecracking tubes or chamber to reduce the resistance to gas flow createdby coke deposits therein is generally undesirable for reactionsinvolving a high heat flux and short residence time in that it reducesthe surface area available for indirect heat transfer at any givencharging rate and it does not cure the sharp decrease in the heattransfer coefficient as carbon deposits accumulate on the heatingsurfaces.

The present invention is directed at minimizing coke formation inhydrocarbon conversion reactions by an im- "ice proved procedure whichdoes not involve accepting undesirable compromises in product yield,on-stream time and corrosion of equipment.

SUMMARY OF THE INVENTION The present invention is a process for theconversion of hydrocarbons characterized by the vapor phase reaction ofa hydrocarbon charge under high temperature conversion conditions whileconfined within a reaction zone boundary surface of metal-ceramicmaterial comprising particles of a catalytically inert, refractory solidmaterial dispersed in chromium. Thus, the reaction vessel is constructedof said metal-ceramic material or the surfaces of the vessel exposed tothe hot reaction mixture are lined or clad with that material whichcontains a refractory solid substance having no substantial tendencytoward catalyzing the formation of carbon in the reaction.

Certain embodiments of the invention are concerned with the thermalcracking of hydrocarbon feedstocks, and it has particular application tothe production of ethylene by the noncatalytic cracking or normallygaseous hydrocarbons containing at least 2 carbon atoms per molecule.Other aspects of the invention relate to indirectly heating endothermicconversion reactions by transferring the heat through the metal-ceramicmaterial to the reaction mixture; the preferred constituents andproportions thereof in said metal-ceramic material as well as preferredreaction temperatures. Other features, benefits and advantages of theinvention will be apparent to those skilled in the art uponconsideration of the following disclosure.

Although the procedure of the present invention is particularly usefulin the thermal cracking of hydrocarbons, it is also applicable to a widevariety of hydrocarbon con version reactions at elevated temperatures,especially where problems are encountered with carburization or cokedeposition. For example, it is contemplated that this technique can beused to advantage in thermal hydrodealkylation reaction, such as thetreatment of toluene or methylnaphthalenes with hydrogen at elevatedpressures and temperatures of the order of 12001400 F. to form benzeneor naphthalene, wherein the rapid and destructive corrosion of reactorsurfaces is often a source of trouble. In the case of noncatalytic orthermal cracking, a wide variety of feedstocks may be processedaccording to the new method, a exemplified by ethane, propane,propylene, butane, isobutane, butylene and its isomers, the butadienes,pentane, cyclopentane, other C saturated and olefinic hydrocarbons,naphthas, kerosines, gasolines, gas olis and heavier hydrocarbon stockswhich can be vaporized for thermal cracking, as well as naphthenic andaromatic hydrocarbons.

Varying reaction severities may be utilized in the practice of thepresent invention. For example, relatively low cracking temperatures inthe range of about 1300 to 1500 F. may be employed with residence timesof about 0.1 to 5 seconds. However, it is usually preferable to utilizeconsiderably higher temperatures in the range of about 1600 to 7850 F.with residence times of about 5 to 200 milliseconds (0.005 to 0.200second) to obtain maximum yields when thermally cracking hydrocarbons atatmospheric or slightly elevated reaction pressures to form olefins. Inproducing acetylenes, higher temperatures ranging up to about 2600 F.are recommended. The tube or reactor temperatures are usually about to300 higher than the aforesaid temperatures of the reaction mixture.

Certain metal-ceramic materials are employed as the confining orboundary surfaces of the reaction zone in the present process. Thereaction vessel or tubes may be constructed from these materials or madeof conventional construction materials (e.g., metals and alloys) whichare clad, lined or coated with one of the metab ceramic materials on theequipment surfaces which are 3 exposed to the hot reaction mixture. Byreason of their low coking tendencies, the metal-ceramic materials described herein are particularly well adapted for operations employing ahigh heat flux and low residence time in re actors wherein the reactionzone may desirably have a relatively narrow cross section as in the caseof small diameter tubes or narrow slots of annular configuration.

Suitable metal-ceramic compositions contains an inert refractorysubstance dispersed as solid particles in chromium metal. The chromiumapparently provides a continuous metal matrix for the dispersedparticles which are of a refractory nature and thus not subject todecomposition at the elevated temperatures involved in the presentinvention, as the dispersed phase component of the metal-ceramicmaterial, excellent results are obtainable with finely divided aluminumoxide, and silicon dioxide or silicon carbide may be used as well asother refractory substances or mixtures thereof suitable for sinteriugwith metallic chromium and which are also substantially devoid ofdeleterious catalytic activity relative to the particular type ofhydrocarbon conversion taking place as well as any associated sidereactions, especially those involving carbon formation. Thus, in thecase of a thermal cracking process, the dispersed component should befree of any significant catalytic effect that promotes either carbonformation or the polymerization of olefins. The refractory material mayconstitute about 5 to 30% or even more of the total weight of themetalceramic composition so long as the chromium content is stillsufficient to provide a continuous chromium phase or matrix; and in thecase of aluminum oxide, a content of between about and is usuallypreferable.

The entire metal-ceramic composition should be substantially free of anyimpurities or minor constituents which are capable of inducing theaforesaid undesired catalytic effects to any significant extent. Forexample, it is considered detrimental in many thermal crackingprocesses, especially those involving extremely high reactor walltemperatures, for the metal-ceramic material to contain any significantamounts of metals of Group VIII of the Periodic Table (e.g., iron,nickel and cobalt) and their compounds in view of the indications hereinthat these substances exert a pronounced catalytic effect upon thereaction mixture.

One example of a suitable metal-ceramic material is Haynes metal-ceramicLT-l and excellent results were obtained in the specific reactionsdescribed hereinafter using LT-l reaction tubes. This material contains23 parts by weight of aluminum oxide in the form of minute particles ofcolloidal size uniformly dispersed in 77 parts of chromium metal. It hasa coefficient of thermal expansion close to that of Type 446 stainlesssteel; hence, the latter may senve as a substrate for some purposes. Inaddition to its desirable lack of catalytic activity, thischromiumalumina composition has very good chemical and physicalproperties, particularly at extremely high temperatures. Its weight tostrength ratio is relatively high and the resistance to thermal shock isbetter than that of ceramic structures. It displays high mechanicalstrength at elevated temperatures, as exemplified by its ultimatetensile strength of 11,700 p.s.i. at 2000 F.; and it retains sufficientstrength for some purposes in continuous operations at 2800 P. wheremost metals fail rapidly. This material has a melting point ofapproximately 3270" F. and it possesses high resistance to abrasion anderosion as well as corrosion by many severely corrosive compounds. Also,it has a relatively high thermal conductivity approximating that of castirona highly desirable factor in high heat flux operations.

The metal-ceramic materials and structural elements utilized in thisinvention may be compounded and formed by various known techniques, suchas slip casting or casting and sintering, and any necessary or desirablefurther shaping may be accomplished by machining with tungstencarbide-tipped tools, drilling with high speed alloy steel drills andfinish grinding using the customary precautions for working hard andbrittle metal alloys.

DESCRIPTION OF SPECIFIC EMBODIMENTS The advantages of the presentprocedure are apparent upon reference to the data set forth in the tablehereinafter which lists some of the operating conditions and resultsobtained in extended cracking runs employing several tubes of differentchemical composition for the cracking of a mixture of propane andpropylene in an approximate :20 weight ratio under generally comparableconditions to produce ethylene. These operations were conducted atatmospheric pressure with the charge gas admitted at ambient temperatureinto 14-inch long cracking tubes of small diameter. The midsection ofthe reactor tube was enclosed within a 40-kilowatt induction heater anda 6-inch length of the cracking tube was heated to provide tube walltemperatures in the 1800l850 F. range by radiation from the metal sleevein the induction heater which was adjusted to maintain a constantinternal temperature of approximately 2050 F. The reaction products leftthe cracking Zone at temperatures in the 1550- 1700 F. range and weresubstantially instantaneously quenched to a temperature below thecracking level while flowing through unjacketed tubes cooled by normalair circulation.

THERMAL CRACKING OF PROPANE-PROPYLENE MIXTURE Stainless Metal-ceramicsteel 77% Cr, Crackmg tube Alloy X 1 type 304 23% A:

Internal diarn., in 0. 18 0. 18 0.28 External diam., in 0.26 0. 25 0. 50Charge rate, s.c.f./hr 1. 5 1. 5 2. 85 Residence time, milliseconds 5353 68 Cracked products:

C2H4, initial wt. percent- 43. 2 48. 9 43. 4 02114, final Wt. percent-45. 4 44. 9 39. 6 Pressure drop in tube:

Initial, in. water 0. 4 0. 3 0.2 Final, in. water. 8 0 7.0 1. 3 Runduration, hrs 14 17 139 A heat-resistant metal alloy commonly used innaphtha cracking tubes and containing essentially 20% chromium, 32%nickel and 47% by weight of iron.

From the tabulated data, it is clearly evident that strikingly superiorresults were otbained in employing a metal-ceramic reaction tubeaccording to the present invention. While good yields of ethylene wereobtained in the runs in the two ferrous alloy tubes designated as alloyX and Type 304, it was necessary to terminate these runs afterrelatively short periods due to the rapid fouling of the tubes withheavy coke deposits as indicated by the twenty-fold increases in thepressure drop through the tubes and further confirmed by observationupon disassembling the apparatus at the end of the runs. The rapidcarbon accumulation is attributed to the catalytic effect of the nickeland iron contents of these tubes. In sharp contrast with this, theduration of the cracking run in the metal-ceramic tube was about tentimes as long while also providing good yields, and total coke depthposition over this prolonged run was still only a small fraction of thatoccurring in the metal alloy tubes as evidenced by the relatively smallfinal pressure drop in the metalceramic reaction tube. The performanceof the metalceramic tube Was a distinct surprise in view of thepossibility of a rapid formation of chromium carbide leading to earlytube failure.

Following the aforesaid run, heavy carbon deposition was deliberatelyinduced in the interior of the same metalceramic reactor tube bycracking undiluted proplyene which has a greater proclivity towardcoking than propane at a drastically reduced feed rate. After thismetal-ceramic tube was decoked in conventional manner by burning out thecarbon deposits, it was then subjected to a second cracking cycle undersubstantially the same conditions, and the pressure drop increased toonly 1.5 inches of water during an operating period of hoursv Forcomparative purposes, the alloy X reaction tube was decoked after the14-hour first cycle, and another cracking run started under similarconditions but this second cycle was terminated after only 2 hours dueto an excessive pressure drop. Both the alloy X and metal-ceramic tubeswere cut or broken in the midsection; and a microscopic examination ofthe cross sections of the tube walls disclosed heavy carburization ofthe alloy X tube but no evidence whatsoever of such attack in themetal-ceramic tube wall.

Extensive operations involving repeated cracking-decoking cycles ofvarious lengths and two different charge stocks were carried outaccording to the present procedure in a 36-inch long metal-ceramic tube(77% chromium==23% alumina) of 0.28" internal and 0.5" external diameterwith a 12-inch midsection of the tube length enclosed within a furnaceoperating at 2100 F. The wall temperatures of the cracking section ofthe tube were in the 1850-2000 F. temperature range and residence timesin the cracking section ranged from 60 to 80 milliseconds. Whenpropane-propylene mixtures containing about 20% by weight of propylenewere cracked at these higher temperatures, the initial ethylene yieldwas typically 42 to 44% by weight of the total reaction products and theruns were usually terminated when the ethylene yield dropped to thelevel of about 35% as the result of the decreased heat transfer andconsequent decreased reaction severity produced by the accumulation of athin layer of coke inside the reaction tube. Even in runs lasting morethan 100 hours, the increase in pressure drop was an acceptable doublingor tripling of the initial value. A more heavily coking charge in theform of 100% propylene was employed in five of these cracking runs witha typical duration of 25 hours, and the initial conversion of propylenewas usually about 72% in producing an ethylene yield of the order of 32%with a gradual decrease to a conversion of 55% and ethylene yield of 22%by the end of the run. No run in this series was terminated by reason ofthe development of an excessive pressure drop in the reaction tube.After 17 cracking-decoking cycles totaling 831.4 hours of exposure tohydrocarbons, there were no indications that the rate of coke buildup inthe 17th cycle was higher than that in the first cycle in themetal-ceramic tube. Nor was there evidence of attack on the tube wallmaterial.

In marked contrast with the above results, it was necessary todiscontinue a series of cracking-decoking cycles in a 36-inch alloy Xreactor tube under comparable conditions as the on-stream or crackingcycle time decreased from 17 hours initially to a few minutes in thefifth cycle as a result of a progresively increasing rate of carbondeposition. It was also evident that the alloy X tube was progressivelymore carburized and otherwise attacked from cycle to cycle.

While the present invention has been described in considerable detail inregard to a few specific embodiments, it is apparent that the practiceof this invention is not restricted to such embodiments and details forit is adaptable to many modifications and variations in its wideapplication to other hydrocarbon reaction mixtures and conversionconditions. Accordingly, the present invention should not be construedas limited in any particulars except as may be recited in the appendedclaims or required by the prior art.

I claim:

1. In a process for the conversion of a hydrocarbon charge in a vaporphase reaction under high temperature conversion conditions in aconfined reaction zone, the irnprovement which comprises confining saidcharge substantially entirely within reaction zone boundary surfaces ofmetal-ceramic material comprising particles of a catalytically inert,refractory solid ceramic substance dispersed in chromium.

2. A process according to claim 1 in which at least a substantialproportion of the heat required for said reaction is substantiallycontinuously transferred through said metal-ceramic material to thereaction mixture.

3. A process according to claim 1 in which said metalceramic materialcontains at least about 5% by weight of a finely divided refractorysolid ceramic substance and sufficient chromium to provide a continuouschromium matrix.

4. A process according to claim 1 in which said metalceramic materialcontains a substantial proportion by weight of aluminum oxide.

5. A process according to claim 1 in which said metal ceramic materialcontains between about 15 and 30% by weight of finely divided aluminumoxide.

6. A process according to claim 1 in which a substantial yield ofethylene is produced by thermally cracking a charge containing a majormolar proportion of hydrocarbon material having at least 2 carbon atomsper molecule at a reaction temperature between about 1600 and 1850 F.without excessive carbon deposition on said metal-ceramic materialduring prolonged operations.

7. A process according to claim 1 in which a hydrocarbon charge iscracked under thermal cracking conditions.

8. A process according to claim 7 in which said metalceramic material isexternally heated to a temperature between about 1650 and 2200 F. forthe indirect transfer of heat to the cracking reaction mixture.

9. A process according to claim 7 in which said metalceramic materialcontains a substantial proportion by weight of aluminum oxide.

10. A process according to claim 7 in which said metalceramic materialcontains at least about 5% by weight of aluminum oxide and sutficientchromium to provide a continuous chromium matrix.

11. A process according to claim 7 in which said metalceramic materialcomprises a dispersion of substantially 23% of finely divided aluminumoxide in 77% by weight of chromium.

12. A process according to claim 7 in which said metalceramic materialcontains between about 15 and 30% by weight of finely divided aluminumoxide.

13. A process according to claim 12 in which a sub- I stantial yield ofethylene is produced by cracking a charge containing a major molarproportion of hydrocarbon material having at least 2 carbon atoms permolecule at a reaction temperature between about 1600 and 1850 F.without excessive carbon deposition on said metal-ceramic materialduring prolonged operations.

14. A process according to claim 1 in which said refractory ceramicsubstance is a compound of the group consisting of aluminum oxide,silicon dioxide and silicon carbide.

15. A process according to claim 1 in which said metalceramic materialcontains a substantial proportion by weight of silicon carbide.

16. A process according to claim 1 in which said metalceramic materialcontains a substantial proportion by weight of silicon dioxide.

References Cited UNITED STATES PATENTS 2,279,260 4/ 1942 Benner et al.106-59 1,973,851 9/1934 Feiler et al. l9647 3,329,735 7/1967 Paul et al.260683 2,477,502 7/ 1949 Utterback et al 260329 1,815,428 7/1931 Blacket al 23252 1,422,878 7/ 1931 Metzger 23-252 2,339,368 1/ 1944 Bagsar23-252 DELBERT E. GANTZ, Primary Examiner J. M. NELSON, AssistantExaminer US. Cl. X.R. 23277; 106-66 mg UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No- 3,536,776 Dated November 11, 1970Inventore-k HO eng I. LO

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2, 11m 21: for "or" reed o1 Column 2, line #5: for "a." read asColumn 2, line 1-9: for "0115" read oils Column 2, line 58: for "TSSOF." read 1850F.

Column 3, line 8: for "contains" reed contain Column line 62: for"reaction" read reactor Column 5, line 15: for read i.e., a. hyphen.

Column 5, line 51: for "progreeively" read progressively 11:12) myULALED JAM 121971 mm]. -u: I 1: m. 8.100 of Pat ents

